As a Group III nitride semiconductor (hereinafter referred to as a nitride semiconductor) has an energy band gap, of a direct transition type, corresponding to visible light to ultraviolet light and can emit light with high efficiency, it is used in products such as light-emitting diodes (LEDs), laser diodes (LDs), or the like. The realization of a white light-emitting diode, where the diode is used in combination with a phosphor, is expected to be a new field of the application of light-emitting diodes.
The output from a light-emitting diode is determined by a product of an internal quantum efficiency, which relates to epitaxial structure and crystallinity, and light extraction efficiency, which relates to resorption in the device and the shape of the device. Among these, resorption in the device, that affects the light extraction efficiency, occurs when light passes through a non-transparent substrate or passes again through the light-emitting layer. As one cause having a great influence on the light extraction efficiency is total reflection at the surface of the device. As is well known, when light is directed toward a layer having a small refractive index from a layer having a great refractive index, light with an angle greater than a critical angle (θc) produces a total reflection at the interface, so that it cannot pass to the layer having a small refractive index.
In the case of gallium nitride (GaN), for example, the refractive index is 2.4, and only light in the Escape Cone, having an apex angle of 24 degrees with respect to the direction vertical to the surface; can be extracted to the outside. The ratio of this is 27%, and this greatly restricts the light extraction efficiency.
In order to avoid restriction of the light extraction at the interface caused by total reflection, there has been known a method for roughening the interface (e.g., see Japanese Unexamined Patent Publication No. 2000-196152) or a method for utilizing an Escape Cone of another surface by processing the shape of the device (e.g., see Japanese Patent No. 2784537).
MOCVD (metal organic chemical vapor deposition) is frequently used for the growth of a nitride semiconductor. MOCVD is a method in which an organic metal and a nitrogen source are reacted on a substrate to grow the nitride semiconductor. However, a single crystal of the nitride semiconductor has not yet been obtained, industrially. Further, although a dummy single crystal substrate, in which a thick-film epitaxial growth is performed on a Si or GaAs substrate with an HVPE (hydride vapor deposition epitaxial method), is commercially available, this dummy single crystal substrate is very expensive. Therefore, a dissimilar substrate such as sapphire (Al2O3), silicon carbide (SiC), or silicon (Si), that is stable at high temperature, is generally used as a substrate for a light-emitting diode.
However, the sapphire or SiC that is a stable material is well known as a material that is hard and difficult to process. Therefore, there is a problem that it is difficult to divide the material to fabricate each device in order to enhance the light extraction efficiency. In the case of a mechanical method by a dicing, a device is frequently broken or cracked, and hence, it is difficult to enhance a yield. Even by means such as a dry etching, that does not use a mechanical method, it takes long time to process, whereby there arises a problem of a remarkably low productivity.
Further, it has been well known that, with the mechanical processing method by a dicing, a layer called a broken layer is formed on the processed surface, which hinders light extraction, and the electric characteristics and optical characteristics are affected by the exposure to high energy particles of plasma even in dry etching.
Wet etching has been known as a processing method causing less damage (e.g., see Japanese Unexamined Patent Publications No. 10-190152 and No. 2000-686084), but the divided cutting surface of each device is vertical to the major surface of the substrate.
Meanwhile, a transparent electrode is used, in most cases, in a light-emitting device made of a nitride semiconductor (e.g., see Japanese Unexamined Patent Publication No. 10-308534). This is because the current diffusion in the lateral direction in a p-type layer is poor compared to an n-type layer.
A comb-shaped electrode, in which negative electrodes and positive electrodes are alternately arranged, may be used for a large chip whose one side is 500 μm or more (e.g., see Japanese Unexamined Patent Publication No. 5-335622). Further, a grid pattern or dot pattern can be employed. A technique for fabricating a positive electrode of a comb-shaped pattern with a transparent material has been disclosed (e.g., see Japanese Unexamined Patent Publication No. 2003-133589).